We present results on the amplifier performance and characteristics of Yb-doped Single Mode fiber amplifiers spanning a broad range of wavelengths from 1028 nm to 1100 nm. Both PM and non-PM amplifiers are discussed, with emphasis on the use of polarization controllers in intrinsically non-PM amplifiers to obtain high Polarization Extinction Ratios (PER). In general, outside the 1064nm region, there has been relatively little discussion or work towards developing high power fiber amplifiers for operation at either 1030 nm or 1100 nm with narrow line-width and high brightness, primarily due to amplifier design and architecture issues related to strong re-absorption and amplified spontaneous emission. Here we address key fiber and amplifier design characteristics aimed at mitigating these issues while highlighting performance attributes and challenges for operation near either end of the above defined spectral range.
Recent progress on high efficiency Tm-doped silica fibers pumped at 790nm has enabled the
demonstration of a 2μm CW fiber laser operating at the 1kW power level and with single mode
beam quality . In addition to this state-of-the-art high power research, Tm-doped fibers are now
starting to find applications in lasers with nsec  and psec  pulsed operation, as well as lower
power CW lasers (50-100W) in the 1940nm wavelength region for medical use .
As with Yb-doped fibers, the question of photodarkening needs to be addressed to ensure the long
term fiber reliability is appropriate for the application. In a recent paper , we proposed that the
up-conversion process in highly doped Tm-fibers was significantly quenched when compared with
lower concentration fibers, under 790nm pumping. This trend was also observed in the
photodarkening rate as measured in a CW 2μm fiber laser cavity operating around 20W output
power. In this controlled experiment, the rate of photodarkening dropped from 15% per 1000 hours
in low concentration fiber to less than 1% in a fiber doped with 4.6% Tm.
In this paper we review the 20W results of our earlier work and then confirm the long term
reliability of 1940nm CW fiber lasers operating at higher (40W) output power, presenting results for
a laser operating for 1200 hours without significant loss of output power (around 12% total power
variation). Over any given 24-hour period during this experiment, the laser operates open loop with
around 8% total power variation, in an air cooled configuration. We believe these results confirm
the appropriate level of long term reliability of Tm-doped fibers for CW fiber lasers at the ~50W
power level, suitable for medical laser applications.
Here we report a compact monolithic 2000nm pulsed laser with a single spatial mode output, ~1.5ns pulse duration, 8kW
peak power and >200mW average power at 20 kHz repetition rate. The gain-switched laser, consisting of a pair of fiber
Bragg gratings and 0.5m of thulium-doped single cladding fiber, was core pumped by a high peak power pulsed 1.5μm
laser. When the input pulse energy of the 20 kHz pump pulses was sufficient enough to saturate the Thulium doped fiber,
a stable 20 kHz pulse train was observed with measured linewidth of 0.05nm which corresponds to the limit of resolution
for the Optical Spectrum Analyzer. This compact, pulsed 2000 nm laser, to the authors' limited knowledge, represents
the first Tm-doped fiber laser with 8kW peak power and several ns pulse duration; which is less than the previously
reported tens of ns pulsewidth previously reported from gain-switched Tm-doped fiber lasers.
We report on an electronically tunable continuous-wave (CW) Ytterbium doped polarization maintaining fiber laser
with more than 100 nm tuning range and more than 10 W linearly polarized single spatial mode laser output. The laser
can be continuously tuned from 1027 nm to 1103 nm without any parasitic lasing peak with the 10dB linewidth of 0.1
nm. The maximum achievable tuning range of the fiber laser was from 1025 nm to 1124 nm with companying parasitic
lasing peaks around 1040 nm. The usage of an intra-cavity acousto-optic tunable filter allows for the fast electronic
tunability with less than 50 &mgr;s response time and random wavelength access capability.
Free-space laser communication has been demonstrated with application potential in many areas such as line-of-sight communications, satellite communications and the last mile solution in a fiber optics networking. Both 0.8 and 1.5 micron wavelengths are currently used in state-of-the-art free space laser communication systems; unfortunately the system performance is imposed by atmospheric turbulence. To reduce the atmospheric effect in free-space laser communication systems, several techniques have been used, such as adaptive optics, aperture averaging and multiple transmitters; however, significant improvement has not been achieved. Theoretically, the seeing effect may be released using a longer wavelength. In this paper, we present a 3.5 micron free-space laser communication system model and its system performance evaluation. A 3.5 micron propagation model based on MODTRAN simulation results in different weather patterns is presented first, and a propagation link budget system model is described after that. The propagation channel performance evaluation results are presented by means of bit error rate versus various propagation distances.
We have performed quantitative theoretical and experimental studies of all-optical self-induced polarization switching in thin films and waveguides of nematic liquid crystals, and demonstrated the feasibility of such processes over the entire visible-near infrared [0.4 - 1.55 microns] spectrum. We have also studied the detailed dynamics of the nonlinear optical interaction with the nematic crystalline axis, and observed interesting frequency selective beam amplification and stimulated scattering effects.
We report theoretical and experimental studies of supra-photorefractive nematic liquid crystals doped with C60 and/or Carbon nanotubes. Theoretical estimate shows that the nonlinear refractive index change coefficient n2 in such systems can be >> 1 cm2/Watt. Experimentally, we have observed n2 of ~ 10 cm2/W, with typical nematic response times.
A detailed theoretical study of nonlinear molecular photonic processes accompanying the propagation of short intense laser pulses through a thin organic liquid cell and an organic liquid cored fiber array is presented. A model is proposed to account for the measurements of a recently developed organic liquid, and a comparison with pure two-photon and excited state absorption mechanisms is performed.
We report on theoretical and experimental studies of all-optical polarization conversion of cw 1.55 μm lasers using nematic liquid crystal in their ordered and isotropic phases. Almost complete conversion of the linearly polarized laser is achieved in a 400 μm thick film at mW power.
We report theoretical and experimental studies of 1-D and 2-d tunable nonlinear photonic crystals made of liquid crystal or liquid crystal infiltrated periodic structures. Theoretical modeling shows that such structures exhibit tunable bandgap, and sugar-prism effect. Experimentally, we have demonstrated the possibility of writing dynamic or permanent [but switchable] index gratings to dye-doped LC films that act as planar waveguides.
We have studied the optical nonlinearities of aligned nematic liquid crystalline films in the near IR communication spectral region (1.55 micrometers ). The measured refractive index coefficients are on the order of 10-3 cm2/W. The origins of the refractive index changes are thermal indexing effect and director axis reorientation. Phase modulation of several (pi) s can be generated with mW-power near IR lasers in micron thick films.